Traditionally, the tongs which are mounted at the end of a slave manipulator arm have been mechanically controlled like other joints of the arm. This has been by means of a bilateral force-reflecting drive trains built into the master and slave arms of the manipulator. Alternatively, the tongs are operated by a cable/spring arrangement in which the spring closes the tongs, and a cable in the slave arm is used to open the tongs against the force of the spring.
Neither of these approaches could be implemented in an advanced servomanipulator being developed at the Oak Ridge National Laboratory which is disclosed in a co-pending application Ser. No. 720,449 filed Apr. 5, 1985 for Advanced Servo Manipulator by William E. Holt et al and incorporated herein by reference thereto.
The cable/spring arrangement did not provide the positive opening and closure that was desired with modular design of the slave arm, and the force-reflection threshold of a backdrivable gear train was unacceptably high and incorporated very large amounts of backlash due to the large number of gear meshes necessary to drive the tongs.